1. Field in the Industry
The present invention concerns an anode for oxygen evolution without forming chlorine in electrolysis of chloride-containing aqueous solutions including seawater.
2. Prior Art
In general, seawater electrolysis is performed to produce sodium hypochlorite by the reaction of chlorine formed on the anode with sodium hydroxide formed on the cathode in addition to the formation of hydrogen on the cathode. For this purpose, there has been used anodes made by coating titanium with an oxide of an element or elements of the platinum group (hereinafter referred to as “platinum group element(s)) as the high performance electrodes.
On the other hand, like fresh water electrolysis to produce hydrogen and oxygen, for production of hydrogen and oxygen in seawater electrolysis, formation of hydrogen on the cathode and formation of oxygen on the anode without formation of chlorine are prerequisite, and hence, a special anode is required.
The inventors found the fact that the oxide electrodes prepared by repeated coating of Mn salt solution together with Mo salt and/or W salt on a conducting substrate and subsequent calcination at high temperatures in air was active as an anode for oxygen evolution in electrolysis of sodium chloride solutions but inactive for chlorine evolution, and disclosed it (Japanese patent Disclosure No. 09-256181). There are two types in this kind of electrodes:
(1) The electrode wherein an electroconductive substrate is coated with the oxide containing 0.2-20 cationic % of Mo and/or W and the balance of Mn.
(2) The electrode wherein an electroconductive substrate is coated with the oxide containing 0.2-20 cationic % of Mo and/or W, and 1-30 at % of Zn and the balance of Mn and wherein the effective surface area of the electrode is increased by leaching out Zn by immersion in hot concentrated alkali solution.
The above-described previous invention is based on the findings that, in production of oxygen evolution anode, calcination of Mn salt coated on the electroconductive substrate leads to formation of Mn2O3 and that inclusion of Mo and/or W in Mn2O3 enhances the oxygen evolution efficiency. In production of oxygen evolution anode, if the calcination temperature is not sufficiently high, stability of the electrode is insufficient due to insufficient crystal growth, but even at high temperatures Mn cannot be oxidized to such a high valence as three or higher because of decomposition of high valence Mn oxide.
Nevertheless, higher valent Mn oxide is expected to have higher activity for oxygen evolution. Thus, an attempt to form Mn oxide by anodic deposition from divalent Mn salt solution was made and gave rise to formation of highly active anode consisting of tetravalent Mn. This finding was also disclosed (Japanese Patent Disclosure No. 10-287991). The electrode based on this finding consists of the electroconductive substrate coated with the oxide containing 0.2-20 cationic % of Mo and/or W, and the balance of Mn, and is characterized in that these oxide are formed by anodic deposition.
Subsequently, the inventors made the following inventions and the inventions were disclosed. They concern the electrolytic cell using the above-described anode (Japanese Patent Disclosure No. 11-256383), the electrode assembly using combination of the electrode and a diode (Japanese Patent Disclosure No. 11-256384) and a method of producing the anode (Japanese Patent Disclosure No. 11-256385), Furthermore, the inventors found that the electrode in which Fe is added to Mn—Mo, Mn—W or Mn—Mo—W oxide was effective as oxygen evolution anode in the solutions containing chloride ion in a wide temperature range up to just below the boiling point of water, (Japanese Patent Disclosure No. 2003-19267). Another patent application was filed for the modified technology of producing the anode including the preparation method of the titanium substrate (Japanese Patent Disclosure No. 2007-138254).
Further research resulted in the finding that addition of Sn to anodically deposited Mn—Mo and/or W oxide improved the activity and durability of the anode, and another patent application was filed in regard to the finding. According to the invention, the anodically deposited oxide consist of 0.2-20 cationic % of Mo and/or W, in which 0.1-3 mol % thereof is substituted with Sn, and the balance of Mn. The anode thus formed showed high performance for oxygen evolution in aqueous solutions containing chloride ion.
In these anodes titanium is used as the electroconductive substrate on which the electroactive catalysts containing Mn are coated. In order to avoid growth of insulating titanium oxide during electroactive catalyst formation by calcination or by anodic deposition and during anodic polarization in electrolysis of chloride-containing aqueous solutions, there has been used electroconductive substrates made of titanium coated with an intermediate layer of the oxide of the platinum group element(s). Formation of the intermediate layer with a sufficient thickness is carried out by repeated coating of a butanol solution containing salt or slats of the platinum group element(s) and subsequent drying followed by calcination in air. Such an electrode made by coating titanium with oxide or oxide of the platinum group element(s) is known as dimensionally stable anode and has been used as the anode for electrolysis and electrodeposition.
For utilization of hydrogen energy, hydrogen production by electrolysis of solutions containing chloride ion without forming chlorine on the anode requires oxygen evolution anodes. However, massive production of hydrogen will result in consumption of a large amount of anode material using intermediate oxide layer of the platinum group element(s). This may cause a problem because of limited resources. Thus, the active electrodes with smaller consumption of the platinum group element(s) are demanded.
The inventors, in view of the preferable characteristics for the coating layer on the titanium substrate that it has the same rutile structure as TiO2 and is stable without being dissolved even under highly oxidizing condition of anodic polarization, and noted that an oxide of tin, SnO2, has the same rutile structure as TiO2 and is stable without dissolution under highly oxidizing condition, hit upon an idea of using SnO2 together with the oxide of the platinum group element(s) in the intermediate layer. Although the electronic conductivity of SnO2 is not sufficiently high, this problem was overcome by the inventors' discovery that the electronic conductivity can be enhanced by addition of Sb, and hence, that it is advisable to use Sn together with Sb.
The electrode based on the above-described idea and discovery consists of a titanium substrate and multiple oxide of the platinum group=element(s), and Sb and Sn. The electrode having the multiple oxide as the electrocatalyst can be used in various electrochemical reactions such as electrolysis and electrodeposition.
More specifically, the electrode according to the invention is an anode used for electrochemical reactions made by coating an electroconductive substrate of titanium with a layer of metal oxide as the electrocatalyst, in which the metal oxide consist of multiple oxide of Sn and Sb, and the platinum group element(s). In this anode the cationic Sn/Sb ratio is in the range of 1-40, and the sum of Sn and Sb in the electrocatalyst is 90 cationic % or less, preferably 1-70 cationic %, and the balance of the oxide of the platinum group element(s). A separate patent application covering this invention was filed.